Telecommunication networks for mobile devices include cellular communication systems; mobile Internet Protocol (IP) networks; paging systems; and others. Cellular systems generally allow mobile terminals to move geographically by “handing off” localized communication links among access points or base stations. Similarly, mobile IP networks allow IP-enabled devices such as wireless Personal Digital Assistants (PDAs) and mobile computers to move about geographically dispersed areas while maintaining a connection to the Internet.
FIG. 1 shows a conventional mobile IP network that covers three service areas SA1, SA2, and SA3. For the sake of simplicity, only IP services are shown, although as explained above, separate transmission networks can be provided for voice services. As shown in FIG. 1, a mobile terminal MT is within service area SA1 served by base station BS1 (also called an access point or AP). Base station BS1 is connected to an access router AR1 which, in turn, connects to an Internet service provider ISP1 that provides access to the Internet. Other base stations such as BS3 may also be connected to access router AR1 such that a common IP address is used for mobile terminals even though the terminals may pass through different service areas. In other words, although there may be a hand off of radio frequency channels when the mobile terminal moves between service area SA1 and service area SA3, it may not be necessary to change the IP address used to communicate with the mobile terminal because the Internet connection is still served by the same access router AR1.
A second service area SA2 is served by a separate base station BS2, which is in turn connected to a different access router AR2. Due to the network topology, access routers AR1 and AR2 use different blocks of IP addresses for communicating with mobile terminals roaming within their associated service areas. If mobile terminal MT moves from service area SA1 to service area SA2, some mechanism is needed to hand off the Internet connection from access router AR1 to access router AR2. Similarly, if service areas SA1 and SA2 are separated by a large logical distance (e.g., AR1 and AR2 are connected to different ISPs), some coordination mechanism is needed to permit data transmitted to a terminal previously operating in service area SA1 to be forwarded to service area SA2 if that terminal moves into area SA2.
One conventional scheme for handing off IP connections is depicted in FIG. 2. Service area SA1 is served by access router AR1, which is designated the “home agent” for communicating with a particular mobile terminal MT. While mobile terminal MT moves within service area SA1, access router AR1 communicates with the mobile terminal using a care-of address. IP packets (e.g., e-mail, Web pages, and the like) are transmitted over the Internet to ISP1, which forwards the traffic to AR1, which in turn forwards the packets to the mobile terminal in its service area. If mobile terminal MT moves to a different service area SA2 served by a different access router AR2, packets that were previously transmitted to AR1 will no longer reach the mobile terminal. One conventional solution is to advertise (e.g., broadcast) the existence of access router AR2 in service area SA2 such that when mobile terminal MT moves into service area SA2, it is notified of the existence of access router AR2, and it receives a new IP address for communicating within service area SA2. Mobile terminal MT or access router AR2 then sends a binding update to home agent AR1 (e.g., through a land line LL or over the Internet), so that home agent AR1 knows the IP address that will allow packets to reach the mobile terminal in service area SA2. The home agent treats this address as a “care-of” address, and all further packets to the original IP address are forwarded to the new IP address. In essence, two separate IP addresses are used to communicate with the mobile terminal: a home agent address and a care-of address that changes at each new point of attachment. This scheme is described in the Internet Engineering Task Force (IETF) Request for Comments (RFC) number 2002 (October 1996)
Advantageously the target access router (AR2) is known by the originating access router (AR1) prior to the handoff (e.g., mobile terminal MT has accepted the advertisement from AR2 and is assigned an IP address for communicating with it). If there are multiple access routers in the target area each with overlapping service areas, there is no easy way for the mobile terminal to select from among them. For example, suppose that a mobile terminal is receiving high bandwidth video data while moving out of a service area. Two other overlapping service areas served by two access routers controlled by two different service providers may be available to accept the handoff of the mobile terminal's IP connection. One of the two access routers may provide high-speed access to the Internet, while the second one may not. There is no way for the mobile terminal to specify or select intelligently from among the two access routers.
Another problem concerns handoff speed. The conventional scenario shown in FIG. 2 may not be able to provide fast handoff speed because of the handshaking required between the mobile terminal and the new access router AR2. Packets may be lost if handoff of the IP connection is not performed smoothly. Moreover, if an IP connection is used for voice-quality signals or music, latency introduced by the handoff may unacceptably disrupt the connection.
Another difficulty with handing off IP connections in mobile networks arises where heterogeneous networks (using different access technologies) served by potentially different (and incompatible) service providers are concerned. Referring again to FIG. 1, if service area SA1 is served by a first Operator while service area SA2 is served by another Operator, then the two service providers must agree on a coordination mechanism to accept handoffs of IP services from each other's system. The problem of providing seamless handovers in IP environments is related to ongoing efforts in the Internet Engineering Task Force (IETF), namely in Seamless Mobility (SeaMoby) and Mobile IP working groups. Context transfer and fast handover protocols have been developed to exchange session-related information or proactively establish mobile IP connectivity, respectively. Both protocols assume that the target access router is known when requesting the desired functionality (see FIG. 1). Although the discovery of the handoff candidate is included in the SeaMoby working group charter, discovery protocols for physically adjacent access routers have not been specified so far. To address at least some of the aforementioned problems a number of proposals for a Candidate access Router Discovery (CARD) protocol are being developed. Advantageously, the CARD protocol is designed to dynamically collect information about neighboring access routers and the capabilities of those routers. By dynamically collecting information about neighboring routers and their capabilities, mobile terminals can dynamically execute a handoff with low latency, and can more intelligently select a target access router. More particularly, in many current proposals for the CARD protocol, access routers maintain a cache of neighboring access routers and associated base stations (i.e., access points). Two access routers are considered neighbors, then, only if the access routers have associated base stations with overlapping coverage areas. The caches are typically populated directly or indirectly in response to actions initiated by mobile terminals.
The CARD protocol is generally susceptible to “denial-of-service” (DoS) attacks by colluding malicious mobile terminals, which cause the erroneous storage of information in the caches of the access routers. And although the protocol requires mobile terminals to be authenticated prior to functioning with access routers according to the CARD protocol, the possibility remains for the mobile terminals to act maliciously by polluting the cache or one or more access routers. Erroneous cache entries can be problematic for access routers and the network in general in a number of different ways.
If the size of an access router's cache is limited, invalid cache entries can eventually replace valid entries. Then, as the number of valid entries is reduced, the effectiveness of the protocol is likewise diminished since necessary mappings between actual neighboring access routers and their base stations will be missing. Such a situation can directly affect the number of mobile terminals that can benefit from the seamless handovers aided by the CARD protocol. Also, if the size of the cache is unlimited, invalid entries can exhaust the memory resources of the access router. Further, in addition to storing the base stations associated with an access router, the cache can also store recent IP capabilities of the neighboring router. These capabilities can be dynamic, requiring frequent updates between the two access routers. A high number of invalid entries though can increase the memory, processing and network load of both access routers, thereby affecting the capacity of each access router to perform other services.
Therefore, what is needed is a system and method for addressing DoS attacks by mobile terminals in the candidate access router discovery.